In powering the magnetron in a microwave oven by way of a power-line-operated electronic inverter-type power supply, in order to achieve an acceptably good power factor in respect to the loading represented by the inverter, as well as in respect to the loading presented to the inverter, it is desirable to extract the power from the inverter by way of a tuned circuit. Otherwise, the Volt-Ampere product that must be supplied by the power line to the inverter, as well as the Volt-Ampere product that must be supplied by the inverter to the magnetron, get to be unacceptably large.
It is particularly desirable to power the magnetron by way of a high-Q resonant L-C circuit wherein the magnetron load is effectively parallel-connected across the tank capacitor of the L-C circuit, and wherein this L-C circuit is effectively series-connected across the inverter's output.
However, when such a high-Q series-excited resonant L-C circuit is not loaded, it acts in effect as a short circuit; which, if allowed to exist for even a very brief period, is apt to cause destructive overload of the inverter and/or the L-C circuit.
In an ordinary magnetron, a cathode must be heated to incandescence before electron emission starts and therefore before the magnetron becomes conductive; and this heating process or pre-conditioning is apt to require from one to two seconds.
Thus, aside from the relatively modest amount of power needed to accomplish the pre-conditioning, a magnetron is substantially a non-conducting load until its cathode has reached incandescence; which implies that, during this brief period of one to two seconds, the short circuit represented by the unloaded series-resonant L-C circuit is apt to cause destructive overload.
One way of preventing such destructive overload is that of connecting in parallel with the magnetron a voltage-limiting means (like a Varistor) characterized by: (i) not conducting at the highest magnitude of voltage normally present across the magnetron when it is conducting; and (ii) conducting heavily at a voltage of somewhat higher magnitude than that.
However, due to the significant amount of energy that must be absorbed by this voltage limiting means, being on the order of 1000 to 2000 Joule for a conditioning period of one to two seconds, the effective cost associated with such a method of preventing destructive overload of inverter and/or L-C circuit is very high.
In respect to the current crest-factor, it is noted that prior-art inverter-type magnetron power supplies provide to the magnetron unidirectional current pulses at the relatively high frequency of the inverter, but with the magnitude of these pulses varying roughly in proportion with the instantaneous magnitude of the power line voltage. The crest-factor resulting from this double-modulation of the magnetron current is particularly disadvantageous.